This invention relates to articles having a surface which is coated with a self-cleaning coating, and more particularly to an improvement on a self-cleaning coating applied to cooking devices.
In the field of cooking devices typified by ovens, it has been popularized to afford the walls or wall liners of the devices a catalytic self-cleaning ability, i.e. the ability of catalytically decomposing oils or greases spattered thereon to gaseous substances. According to U.S. Pat. No. 3,266,477 to Stiles, a self-cleaning wall is provided, for example, by forming a ceramic coating on the wall and applying oxidation catalyst particles to the surface of the coating such that the particles are partly embedded in the coating but mostly exposed to a cooking space. However, this method is considered to be unsuitable to practical applications since the catalyst particles are easily damaged by abrasion for various reasons during use of the devices, and hence the coating cannot retain its self-cleaning ability for a satisfactorily long period of time.
U.S. Pat. No. 3,547,098 to Lee proposes, as an improvement on the method of the Stiles patent, to smelt a mixture of an oxidation catalyst and a glass frit thereby to incorporate the catalyst homogeneously into the glass matrix. According to this patnet the mixture is smelted at a temperature above about 1200.degree. C., i.e. above a vitrification temperature, followed by quenching in cold water to obtain a catalytic frit. A self-cleaning coating produced from the resultant frit is characterized in that the catalyst is fixed throughout the coating in a permanent manner, whereby the coating is highly resistant to abrasion and excellent in durability.
However, this method also has some shortcomings. As the most serious problem, the heating of the catalyst to a temperature above 1200.degree. C. almost inevitably causes the activity of the catalyst to lower. At such a high temperature a metal oxide utilized as the catalyst readily reacts with a certain ingredient of the frit to turn into a stable compound, whose catalytic ability is only a fraction of that of the initial catalytic oxide. The uniform distribution of the catalyst (or its derivative) in the frit offers another problem that a metal base to be coated must be lined with a ceramic undercoat beforehand since firing of the catalytic frit directly onto the metal base tends to cause corrosion of the metal base by the action of the catalyst at a high temperature, e.g. about 800.degree. C., necessary for firing. As a still another problem, firing of the catalytic frit onto the undercoat needs to be accomplished at a relatively high temperature, e.g. at about 700.degree. C. or above, because the frit comprises a considerably large amount of catalyst and hence is rather poor in its adhering property. The employment of such a high firing temperature causes augmentation of dimensional errors of the metal base and as a consequence an increase in the number of unacceptable products.
U.S. Pat. No. 3,587,556 to Moreland teaches to form a self-cleaning coating by the steps of preparing a slip in which a glass frit is mixed with a powdered oxidation catalyst, applying the slip onto a metal base and firing the applied slip. The resultant coating has a porous matrix layer formed of the frit particles partially fused to bond to each other, and the catalyst particles are dispersed throughout this matrix layer. This type of self-cleaning coating is superior to the catalytic coating of the Stiles patent both in resistance to abrasion and durability of the self-cleaning ability, and is considered to be advantageous over the coating according to the Lee patent since the catalyst is not heated at such a high temperature as will cause lowering of its catalytic activity.
Besides, it has been proposed to utilize an alkali metal silicate in place of, or in combination with, a glass frit as the support for oxidation catalysts.
Thus, it is deemed a standard technique in the art of self-cleaning coatings to disperse a powdery oxidation catalyst uniformly in a porous matrix layer formed of an inorganic binder.
However, certain problems about this type of self-cleaning coatings remain unsolved.
One of such problems is that the oxidation catalyst, which is usually a metal oxide (or metal oxides), in the coating undergoes gradual reduction as the result of oxidation decomposition of oils spattered on and diffused into the coating and/or by the reducing effect of gas burner flame in the cooking device. The reduction of the catalyst results in lowering of its catalytic activity and, besides, in most cases accompanies a change in the color of the catalyst, meaning the occurrence of local and irregular discoloration of the self-cleaning coating which of course impairs the appearance of the coating. The Stiles patent proposes to revive (oxidize) the partially reduced catalyst on the coating by treatment with a solution of a strong oxidizing agent such as hydrogen peroxide, but it will be impossible to accomplish such a treatment in the home. This problem must be solved at the production stage.
Another problem is that conventional self-cleaning coatings require relatively high temperatures, e.g. about 300.degree. C., to exhibit their ability to satisfaction, whereas such high temperatures are rarely realized in cooking devices. Actual cooking temperatures in most of current cooking devices seldom exceed 300.degree. C., and sometimes the wall surfaces in these devices remain at temperatures considerably lower than an established cooking temperature, e.g. at about 250.degree. C. or even below when the cooking temperature is 300.degree. C. Oxidation decomposition of fatty acids, main components of oils and greases spattering onto a self-cleaning coating, proceeds rather slowly at temperatures realizable in cooking devices. For example, it takes about 30 minutes at temperatures of 250.degree.-300.degree. C. Even when a cooking temperature of 250.degree.-300.degree. C. is employed, in many cases heating is stopped before the lapse of 30 minutes after spattering of oils onto the self-cleaning coating, so that a considerable portion of the spattered matter remains undecomposed and adhered to the coating. Accordingly, there is a tendency that increasing quantities of oils or soils accumulate on the coating during repeated use of a cooking device, so that the accomplishment of a thorough decomposition of the spattered matter becomes more and more difficult. The difficulty is further augmented when the coating is significantly covered with oils or soils since in such a state the catalyst is not sufficiently supplied with air.
To summarize, conventional self-cleaning coatings are not yet fully satisfactory from a practical viewpoint.